1. Field of the Invention
The present invention relates to a capacitive type touch sensor device. Further, the present invention relates to an electronic apparatus which is provided with the touch sensor device.
2. Description of the Related Art
The touch sensor device is a device which detects position coordinates pointed by using an indicator such as a fingertip or a pen or detects existence of pointing actions. Normally, it is used in combination with a surface display device such as a liquid crystal display (referred to as “LCD (Liquid Crystal Display)” hereinafter), a plasma display (referred to as “PDP (Plasma Display Panel)” hereinafter), or an organic electroluminescence display (referred to as “organic EL display” hereinafter).
A user-friendly man-to-machine interface can be achieved by controlling the display content of the display device and controlling the apparatus with a computer through inputting the output of the touch sensor device to the computer. Currently, it is broadly used in everyday life such as for game machines, mobile information terminals, ticket-vending machines, ATMs (Automatic Teller Machines), car navigation systems, and the like. Further, in accordance with the improvement in the performance of the computers and spread of the network-connected environments, services supplied with electronic apparatuses are diversified. In accordance with that, the needs for the display device provided with the touch sensor device are continuously expanding.
As the types of the touch sensor device, there are a capacitive type, a resistance film type, an infrared ray type, an ultrasonic type, an electromagnetic induction type, and the like are known. Among the touch sensor devices, the capacitive type touch sensor device is capable of detecting a contact of an indicator through thin glass, plastics, and the like and capable of detecting the touch even when it is not pressed strongly. Thus, the capacitive type touch sensor device exhibits the excellent resistance for repeated inputs (contacts). Because of such characteristics, the capacitive type touch sensor device is broadly used in many application regions such as for industrial products and white household appliances.
The capacitive type touch sensor devices are classified into a projected capacitive type and a surface capacitive type.
The projected capacitive type is a type in which X-Y transparent electrodes are formed in a matrix form. The X transparent electrodes and the Y transparent electrodes are formed via glass and an insulating layer. When an indicator approaches the X-Y transparent electrodes, the capacitance between the electrodes is increased. A controller detects the change in the capacitance in the X-Y line, and detects the position of the indicator.
In the meantime, the surface capacitive type is constituted with an insulating transparent substrate, a uniform transparent conductive layer formed on the surface thereof, and a thin insulating layer (a protection layer) formed on the top surface thereof. When driving the touch sensor device, an alternating current voltage is applied to the four corners of the transparent conductive layer. When the surface of the touch sensor device is touched by an indicator via the capacitance formed by the transparent electrode layer and the indicator, a small current is flown into the indicator. The small current flows to the point at which the indicator touches from each of the four corners of the transparent conductive layer. Then, a signal processing circuit detects existence of a contact from the sum of those currents. Further, the coordinate of the touched position is calculated from the ratio of those currents. Note here that the current flown in each corner of the transparent conductive layer is converted to a signal proportional to the current by the signal processing circuit. The technique regarding such surface capacitive type is disclosed in Japanese Examined Patent Publication Hei 01-019176 (Patent Document 1), for example.
However, when the surface of the touch sensor device and a fingertip makes a contact, the entire hand or the arm (referred to as “palm” hereinafter) as a part of the human body other than the fingertip actually comes in close to the surface of the touch sensor device. Thus, a capacitance is also formed between the transparent conductive layer and the palm. When the capacitance is formed also between the transparent conductive layer and the palm as described above, a current generated according to the approach of the palm flows and is superimposed on the current detected by the signal processing circuit. Thus, the detected position is shifted from the position coordinate indicated by the fingertip when the detected position is acquired as it is. For example, techniques for improving the accuracy of the touched position on the touch screen are disclosed in Japanese Patent Application Publication 2008-543226 (Patent Document 2) and Japanese Unexamined Patent Publication 2012-104102 (Patent Document 3), for example. Patent Document 2 discloses a technique which corrects the position according to the chronic characteristic of the signal before and after a contact of a fingertip.
Patent Document 3 discloses a technique which estimates a signal generated according to the approach of a palm based on the tendency of the signal changes immediately before a fingertip touches the surface of the touch sensor device, and calculates the position by subtracting the estimated signal generated according to the approach of the palm from the acquired entire signals.
Japanese Unexamined Patent Publication 2012-146026 (Patent Document 4) discloses a projected type touch panel. Depicted therein is that the detected touch position is shifted due to the influence of a capacitance generated by the approach of a palm, specifically the untouch part that is the part from the second joint of a finger to the base part. In order to overcome such inconvenience, the tilt direction and tilt angle of the indicator are estimated from the extent of the generated capacitance of the sensor whose generated capacitance is the greatest and the extent of the generated capacitance of the peripheral sensors to correct the generated capacitance of the sensor. Note, however, that a plurality of sensors are required for employing the technique depicted in Patent Document 4 as in the case of the projected capacitive type. That is, it is necessary for the plurality of sensors formed by the transparent conductors to be separated from each other. Therefore, the technique depicted in Patent Document 4 cannot be employed for the surface capacitive type that has a single sensor.
The following analysis can be given based on the viewpoint of the present invention. With the detected position calculation method depicted in Patent Document 3, the position accuracy is improved for the operation called a tap which is an action of lightly hitting on the surface of the touch sensor device by a fingertip and an operation of pressing the screen with a fingertip and setting still for a while. However, when a finger is slid while making a contact with the screen by the fingertip, i.e., a drag operation, the shift of the detected position becomes greater as the touched position touched down by the finger leaves away from the finger position. Thus, the position accuracy regarding the drag operation is rather deteriorated compared to the case where the technique depicted in Patent Document 3 is not employed.
In a case where the technique of Patent Document 3 is employed, the reason why the position accuracy is poor with the drag operation is analyzed as follows. When a drag operation is conducted after a touch, the position coordinate of the fingertip changes on the touch screen. The four touch signals acquired by the signal processing circuit change in accordance with the change in the position coordinate of the fingertip. However, the position of the palm also changes, so that the position of the capacitance formed between the touch sensor (the transparent conductive layer) and the palm on the touch screen changes as well. Therefore, the signals generated in accordance with the approach of the palm superimposed on each of the four signals acquired by the signal processing circuit also change. That is, during the drag operation, both the signal generated according to the contact of the fingertip and the signal generated according to the approach of the palm change. Thus, the detailed breakdown of those is unknown. However, in Patent Document 3, the detected position is acquired by using the changing signal acquired by the signal processing circuit and the estimated fixed-value signal generated according to the approach of the palm. Therefore, the difference between the signal generated according to the approach of the palm estimated immediately before a “touch-on” judgment and the signal generated according to the approach of the palm after the drag operation is conducted and the position is changed deteriorates the position accuracy.
Therefore, the issue to be overcome by the present invention is that the position accuracy at the time of the drag operation is poor due to the influence of the approach of the palm.